Intake or induction valves for internal combustion engines are normally of the poppet type valve. Each such poppet valve includes an annular head adapted to seat against a valve seat in the cylinder head of the engine to control the flow of induction fluid through an intake passage in the cylinder head to a cylinder of the engine during the induction cycle for that cylinder. Normally the annular head of such a poppet valve is constituted as a body of revolution about an axis and is provided with a stem extending therefrom which is co-axial with this axis. The stem of the poppet valve is reciprocably journaled in a suitable valve stem guide bore provided in the cylinder head. The free end of the stem of the valve normally projects from the cylinder head a suitable distance so that it can be engaged by a suitable actuator means, such as a rocker arm, whereby to effect reciprocating movement of the poppet valve in an opening direction. A valve return spring is used to effect reciprocating movement of the poppet valve in an opposite or valve closing direction.
In such a conventional poppet type valve, when the head is unseated from the valve seat, a uniform annular flow area is provided between the valve head and the inlet port for the flow of induction fluid into the combustion chamber for that cylinder of the engine. With such an arrangement, induction fluid flow is substantially uniform around the head of the valve and, accordingly, depending on the location of the inlet port relative to the longitudinal axis of the cylinder, there is little or no swirling motion of the induction fluid entering the cylinder around the longitudinal axis of the cylinder.
Because of this, various means have been used in the prior art to promote swirl of the induction fluid on entry into the combustion chamber, such as for example, by the use of a shrouded valve or a swirl port. By way of an example, by providing the outer surface of the valve guide in the inlet port with suitable shaped ribs or vanes, induction fluid entering the cylinder from this port is given a direction of flow which, in general, is in the desired direction of induction fluid flow rotation around the axis of the cylinder. However, it has been found that whether a shrouded valve or a swirl port is used to introduce swirl, the use of such a swirl inducing device can effect a reduction in the volumetric efficiency of the engine.